Humoral immunodeficiency after ABMT may worsen the course of infectious complications as already described in this clinical setting; children with low Ig values of the three isotypes during the first week after ABMT experienced more severe infections during the procedure than those with normal values. The aim of the study was to establish the prevalence, the duration and the risk factors of Ig deficiency after ABMT. Serum Ig levels of 160 children treated with high-dose chemotherapy (HDCT) followed by ABMT for solid tumors were studied prospectively before HDCT and weekly from the day after transplantation until the patients were discharged from the unit, as were the associations of the following covariates: patient characteristics, previous conventional chemotherapy (CCT), conditioning regimens, marrow graft and complications following ABMT. Serum Ig deficiency for at least one isotype was already present before HDCT in half of the children and mean serum Ig values decreased after HDCT. Serum Ig deficiency was early (day 7), inconstant, heterogeneous (IgM deficiency was more frequent and lasted longer) and brief (< 1 month). Children with low Ig values before HDCT were at high risk of profound and prolonged humoral immune deficiency. Previous CCT with more than six different drugs was the main risk factor for low serum IgM values before HDCT, on day 7 and on day 21 post-HDCT. This study shows that Ig replacement therapy could be useful after ABMT provided it is given to the patients defined on the basis of these specific risk factors and serum Ig levels before HDCT.